1. Field of the Invention
The present invention relates to an objective lens system for use within a microscope which is employable in ultraviolet range, in particular, in the far ultraviolet range, where light has a wavelength shorter than 300 nm.
2. Description of the Background Art
It is commonly known in the art that a microscope has the property that, assuming a constant numerical aperture (NA) of the objective lens system within the microscope, the shorter is the wavelength of light used therein, and the better the resolution. Thus, it is possible to observe sample in greater detail by shortening the wavelength of the illumination light. In addition, illuminating a sample with ultraviolet light often results in fluorecence of a stronger intensity being discharged from the sample than obtained by illuminating with visible light. Therefore, a microscope employable in ultraviolet range is preferred in the art, because more information is obtained by observing a sample through such a microscope. Thus, the objective lens system for use in such a microscope must work in the ultraviolet and/or far ultraviolet range.
One of the known conventional objective lens systems which works in the ultraviolet and/or far ultraviolet range and usable as an objective lens system for use within a microscope is described in "Hikari Gijyutsu Contact," Volume 25, Number 2, Page 137 (Feb. 1987). That objective lens system is illustrated in FIG. 1.
In FIG. 1, an objective lens system 170 includes a first lens 171 made of fluorite, a second lens group 172 and a third lens group 173 disposed in that order from an object side (left-hand side of the figure) to an image formation side (right-hand side of the figure). The second lens group 172 includes two convex lenses 172b and 172c both made of fluorite and a concave lens 172a made of quartz. The second lens group 172 is formed by holding the concave lens 172a between the convex lenses 172b and 172c and joining the same to each other. The third lens group 173 is formed, in a similar manner to the second lens group 172, by holding a concave lens 173a made of quartz between two convex lenses 173b and 173c both made of fluorite and joining the same to each other.
Since the lenses 171, 172a to 172c and 173a to 173c are made of either quartz or fluorite, the objective lens system 170 is capable of transmitting and is usable in the ultraviolet and/or far ultraviolet range.
In addition, chromatic aberration can be corrected in the objective lens system 170, since the second lens group 172 is composed of the concave lens 172a made of quartz and the convex lenses 172b and 172c made of fluorite while the third lens group 173 is composed of the concave lens 173a made of quartz and the convex lenses 173b and 173c made of fluorite.
The convex lens 172b, the concave lens 172a and the convex lens 172c of the second lens group 172 are brought into optical contact and joined to each other. Similarly in the third lens group 173, the convex lens 173b, the concave lens 173a and the convex lens 173c are brought into optical contact and are thereby joined to each other. This is attributable to the current technical level that has not been able to provide adhesive which transmits far ultraviolet light. Further, when the junction (contacting) surface between lenses has to completely eliminate reflection thereat, there is no option other than to cement by optical contact. Thus, in the process of manufacturing the objective lens system 170, junction surfaces must be finished with extremely high accuracy, which result in much higher costs.
The inventor herein has already suggested an objective lens system for use within a microscope in which the aforementioned problem is solved. See Japanese Patent Laid-Open Gazette Nos. 1-319719 and 1-319720, referred to below as the "prior applications." FIG. 2 shows an objective lens system for use within a microscope, namely, objective lens system 160, according to an embodiment of the prior applications. The suggested objective 160 includes lenses 161 to 163, which are made of either quartz or fluorite. The lenses 161 to 163, i.e., a first to a third lenses, are displaced in that order from an object side (left-hand side in the figure) to an image formation side (right-hand side in the figure) with preselected air spaces therebetween. This enables the objective lens system 160 to be used in both the ultraviolet and far ultraviolet range. The lenses 161 to 163, as has just been mentioned, are separated from each other; that is, the objective lens system 160 includes no junction surfaces. The objective lens system 160 is therefore free from the high manufacturing cost problem.
The objective lens system 160 cooperates with an image formation lens system (described later) in order to form an image of an object to be observed on the focal plane of the image formation lens system at a predetermined imaging magnification M. The imaging magnification M is a ratio of the focal length f.sub.2 of the image formation lens system to the focal length f.sub.1 of the objective lens system 160, and is given as: EQU M=-f.sub.2 /f.sub.1 ( 1)
In general, one changes the objective lens system while retaining the image formation lens. The imaging magnification M is thereby changed. Objective lens systems for replacement are necessary for this end, each of the lens systems having a focal length different from the focal length f.sub.1.
According to equation (1), a replacement objective lens system which has a focal length of 2.multidot.f.sub.1 is necessary in order to halve the original imaging magnification M. If what is required here is nothing more than to form the replacement objective lens system such that its focal length is 2.multidot.f.sub.1, the required objective lens system merely has to have a size of a proportionally enlarged version of the objective lens system 160.
However, if the objective lens system 160 is replaced with the required objective lens system for replacement mentioned above (which is equal to the objective lens system 160 doubled in terms of size), the distance between the required objective lens system for replacement and the object to be observed would also have to be doubled as long as the pupil of the microscope is fixed. This is extremely time-consuming as well as labor-consuming in that the microscope must be brought into focus once again all from the beginning after the replacement, enormously adversely impacting the operation of the microscope. In addition, such replacement causes doubling of the pupil size, which in turn causes a remarkable change in the quantity of light illumination the object. On the other hand, if the position of the object is fixed, the position of the pupil has to be moved. This should also be avoided in an illumination system for illuminating the object, since positional changes of the pupil exerts unfavorable effect upon the illumination conditions.
Thus, when imaging magnification is to be halved by replacing the objective lens system, the replacement objective lens system must have:
(a) a focal length double that of the objective lens system 160;
(b) parfocality; that is, the property that it eliminates the necessity of bringing the microscope into focus once again after the lens replacement; and
(c) a pupil of roughly the same as that of the objective lens system 160.
The equation (1) also shows that an replacement objective lens system with a focal length of 5.multidot.f.sub.1 is necessary to observe an object at an imaging magnification of M/5. Thus, in the case of obtaining an image of the object under a microscope at M/5 power, the replacement objective lens system must have:
(a) a focal length five times larger than that of the objective lens system 160;
(b) parfocality; that is, the property of eliminating needing to bring the microscope into focus once again after lens replacement; and
(c) a pupil which is roughly the same in size as that of the objective lens system 160.